Tanning skins or hides



Patented Oct. 8, 1935 TANNING SKINS OR RIDES Clarence K. Reiman, Newton, and Frank Wayland, Salem, Mass.

No Drawing. Application May 2, 1932,

Serial No. 608,850.

16 Claims. (01. 149-5) This invention relates to the art of tanning skins or hides in the production of leather and consists of an improved method or process'of tanning. It also includes within its scope .5 leather having new and improved characteristics and herein shown as produced by the method of our invention.

In the copending application of Frank Wayland, Serial No. 586,175, filed January 12, 1932,

10 is disclosed a method of tanning under tension in a concentrated solution of chrome or other metallic'tan, with the object of producing leather of satisfactory quality and at the same time obtaining the greatly increased yield which is the 15, major advantage of the tension tanning process.

In one aspect, the present invention consists in a further development of the invention of said application with a view to improving tension tanning, and in another aspect it consists in improvements which may be advantageously applied to tanning in general.

Among the objects achieved by the present invention are the conservation of tan, reduction in time of tanning, production of leathers of high chrome content without undue waste of tan, elimination of the pounding and drumming to which chrome tanned leathers have been heretofore subjected while tanning, reduction of handling and operating costs, reduction in actual losses of hide substance as compared to those formerly caused by drumming, reduction of shrinkage in area, and an apparent or actual increase in thickness of the tanned stock.

In the ordinary methods employed today in chrome tanning, a weighed amount of stock (hides, sides, grains, splits or skins) is placed in a drum, wheel, paddle or other container, and then a definite amount of the tan liquor is allowed to enter the system either all at once, or 49 in several shots, or the liquor is run in slowly during the first part of the tanning period. The total amount of tan thus contacted with the stock is approximately the amount that is to be taken up by the stock, that is, only a necessary excess is used, and in this country this excess is discarded at the end of the tanning period of the particular run of stock in question. In Europe, this small excess of dilute tan liquor is left in the wheel, or equivalent, andthe next run of stock is thrown into this liquor and then, after a short period during which much of this residual tan'is picked up by the fresh lot of stock,

the residual liquoris discarded and fresh tan 1 liquor runinto the, wheel in order to complete the tanning.

Inorder to use up as large a part as possible of the tan so employed, and to distribute it uniiormly throughout the stock, and to cause the chemical reaction between the stock and the rapidly decreasing concentration of ten in the liquor 5 to take place as rapidly asv possible, the stock and tan liquor are vigorously contacted by tumbling around in the wheel or drum, or to a somewhat less extent by running in the paddle wheel. It

should be pointed out that the paddle wheel method, due to the lower intensity of the agitation. requires a longer tanning period and leaves a larger part of tan in the resldual'liquor than does the wheel method with its very vigorous tumbling and pounding. When a long liquor is used-enough practically to float the hides--, and as is always used in paddles, the time of agitation may be five or more hours, and following this period the hides are generally left immersed in the residual liquor until the next day. Thus, not only do the hides receive a tremendous amount of very rough treatment, but the tannery equipment is tied up for long periods.

Tanners have begun to realizethat all this tumbling and pounding is exceedingly detrlmen- 5 tal to the stock, and a compromise is frequently -eifectedthe wheels or paddles being operated only part of the total tanning period and at intervals--and this reduces toa certain degree the damage which this pounding admittedly does. Recently a short tan process has met with limited favor in this country, which seeks to avoid or diminish this difficulty. In that process the moist hides, properly prepared for tanning, are thrown into the empty wheel or drum and a small volume of more concentrated tan liquor containing approximately the amount of tan that it is desired to combine with the stock is run into the wheel, and the wheel is turned for a shorter period, two hours or sometimes a little less. In this process the volume of liquor is barely enough to wet the hides, and at the end of the tanning period there is only about a gallon of liquid in. the wheel after the hides have been removed, and it can be readily seen that the pounding which the hides receive in this case, while for a shorter period. is necessarily much more severe because there is practically no liquid in the drum to act as a cushion. This is a matterof much importance when it is recalled that these drums are from 8 to 12 feet in diameter and 1000 pounds or more of stock are placed in the drum at one time. In this short tan process it should be pointed out that a relatively concentrated solution of tan is used, since about the same total amount of tan must be put into the hide whether the quantity of liquor is large or small in amount. This concentration is about 1 pound per gallon of chromic oxide (ClzOa), or when Tanolin, a well known commercial prepared tan is used, about 4 pounds per gallon of said Tanolin. However, this relatively high :oncentration obtains only at the start of the tanning period, the tan is rapidly absorbed from this concentrated solution and it rapidly becomes dilute and the rate of absorption of the tan by the hide diminishes, until, towards the end of the tanning period, when it is becoming more and more difficult for the almost tanned hide to take .up more tan, the solution is very dilute and tanning practically ceases. Probably almost an equilibrium between the small volume of dilute tan liquor and the tanned hide taking place. In other words, at the end of the tanning period when a high concentration of tan should be used if more tan is to be forced into the hide, the concentration of tan is allowed to fall to the lowest point that it reaches during the entire process. It willbe readily understood that this same condition obtains when a "long liquor is used, and in fact is generally true throughout the tanning industry wherever chrome tanning is done.

This condition of equilibrium, or the approach thereto, between the tanned stock and the dilute residual tan liquor is one of the reasons why leathers of high chrome content are so seldom produced in this country. Under the previous practice, if larger amounts of tan are put in contact with the hides, a larger and larger amount of residual tan is discarded, and this is one of the difficulties which our invention makes it possible to avoid, that is, to produce leather of high chrome content (5 or 6% or much higher) without wasting chrome tan.

Another factor-the acidity of the tan systemwhich has a very important bearing on both the chrome content of the leather and the wastage of chrome tan, is involved in the improvements of our invention and is discussed below.

Thus, it will be seen that in the chrome tanning methods in use heretofore in this country and abroad, there are a number of serious difiiculties which up until now have proven impossible of correction, and which may be enumerated as follows: Chrome tan is wasted, leather of high chrome content is seldom made and then "only at a high cost, equipment is tied up for long periods, and most important of all, the tumbling and pounding, which has been found necessary heretofore in order to bring about the combination of the chrome tan and the hide substance, does untold harm to the leather by breaking fibres, by developing internal heat so that some of the collagen is converted to soluble gelatins and the like and, therefore, represents so much hide substance wasted, by making the leather "mushy and lifeless-breaking it up, by causing an undue amount of shrinkage in area, besides the actual loss in weight noted above, and by producing as leather from the tan wheel a wrinkled and creased product which requires many subsequent operations and a deal of time and labor to convert to the relatively smooth and flat material which the trade will accept as satisfactory finished leather.

In tanning leather by our new and improved method, we eliminate all drumming and rough handling during the tanning operation, tan in a short period of time, and produce at will leather of as high chrome content as desired, and without undue waste of tan.

We have discovered that all these valuable results may be achieved by immersing or otherwise contacting the hide, grain, split, side or skin in, or with preheated concentrated tan liquor in a quantity enormously in excess of that re- 5 quired for the actual tanning process, for a relatively short periodi'or example, for about one minute to minutes, depending, among other things, on the type of skins, the concentration and temperature of the tan liquor and the amount 10 of chrome desired in the finished leather. Light sheep skins have been fully tanned by our novel method with 30 seconds immersion, whereas heavy belting stock may require the longer period.

In carrying out the method of our invention, 1 the considerations determining the proper acidity of both pickled stock and tan liquor are so different from standard procedure that a discussion of this phase of the invention is desirable. When pickled stock containing acid is placed in this 20 large volume of concentrated heated (100 to 1''.) tan liquor, not only does tan penetrate rapidly throughout the thickness of the stock.

but much of the acid originally in the pickled stock diffuses out into the tan solution, If as pickled cowhide grains are left in this liquor (4# Tanolin/gal.) for 30 to 60 minutes, for example, the acidity of the stock after removal from the liquor will be found to have greatly changedfrom a pH of between 1 and 3 to a pH of between 30 3.0 and 4.0, for example. In general, the change is greater the longer the time of immersion and the higher the basicity of the liquor.

It is generally understood that the pickledskin-tan system must be relatively high in acid as (low pH) at the start of the tanning process, but that the stock out of tan and ready for washing must be less acid (higher pH) so that the tan will actually combine with the hide substance and not wash out. That is, the more acid the 40 system the more rapidly and more readily the tan penetrates, but on the other hand the more acid the system, the less the actual combination of the chrome tan with the stock, and the greater the amount of alkali, generally bicarbonate of 45 soda, which must be used in order to reduce this acidity and bring about this combination and so set the tan. Tanners everywhere recognize the danger of using large quantities of the bicarbonate of soda and in the best practice the mini- 50 mum quantity to do the necessary work is employed, and the best leather is made (or rather would be) when no bicarbonate is used or is necessary. The careless use of bicarbonate produces a coarse and uneven grain which at times 55 is so marked as greatly to reduce the quality of the finished leather. Thus in the prior art 1 methods the tanner must choose his acidity conditions with care or he will be in one of the two diiliculties-slow and imperfect penetration with so an overtanned and brittle grain, if his system is not acid enough, and poor quality leather if his systemistooacid and he has tousemore than a small amount of the bicarbonate of soda.

Not only will the pH of the tanned stock be as increased (acidity reduced) without bicarbonate additions to the system of stock-plus-tan-liquor, but other steps can be taken to further this advantage. For example, alum can be used with the acid and salt in making up the pickle liquor, 70 as is .well understood by those well versed in the art, and it will be found that two runs of stock using the same tank of tan and same temperature and other conditions, and pickled with the same per cent of both salt and acid but in one case 75 IRE/matureCHEMICAL MODIFI- CANON F TEXTILES d FlBERS,

using a percentage of alum in the pickle will produce different results-the alum pickled stock will leave the tan vat at a higher pH than the other lot and will require less bicarbonate in the final neutralizing and washing and will show a finer break, and therefore, superior quality leather. A similar result will be obtained if other buffering materials are used in the pickle or if the two tan vat system is used as discussed below. The secret of success in this matter resides in having the system pickled-stock-tan-liquors so arranged that at the start of tanning the system (within the skin) is acid (low pH) and at the end of the ztganning period more nearly neutral--pH about For example, when pickled stock of normal acidity is used, having a pH of l to 3, and which has been pickled in a liquor containing perhaps 1% of sulfuric acid and 5 to 10% of salt based on the limed weight, and when the tan liquor (iii/gal. of Tanolin R) has a normal basicity, perhaps 0.80 to 0.95, where basicity is defined as ratio of CI2O3 over H2504, we have found that less bicarbonate of soda is required than is normally used for the final neutralization of the stock. The reason for this is apparentin the prior art processes a small volume of tan solution is used and as a considerable part of the acid present in the pickled stock washes out into the tan solution this small volume of solution becomes correspondingly more acid, the basicity of the solution drops, too small a proportion of the tan combines with the stock, and in order to fix or set the tan in the leather, a definite quantity of the alkali is required. Part of this is generally put into the wheel or paddle towards the end of the drumming period, thus precipitating much of the small amount of tan that is later to be discarded, and causing some of this excess tan to be either precipitated onto and in the grain layer or to be drummed in after precipitation, and causing thereby a harder, weaker and more brittle grain than is desired.

In accordance with our improved method with the large volume of tan liquor the acid which washes out of the pickled stock is diluted in the larger volume to such an extent that it has practically no effect, the basicity remains substantially that of the tan solution, and where a normal basicity, about 0.80 to 0.95, is used no bicarbonate additions to the tan liquor are necessary or desirable, and in fact stock after 30 minutes or more in the tan vat will have a pH of 3 or higher. Even after washing we have frequently found it unnecessary to add bicarbonate at the point where chrome tanned stock is regularly neutralized, and when some neutralization is required, we have found the amount of bicarbonate of soda necessary to be less, about of that normally required.

It will be noted from the above that the acidity of the tan solution used in carrying out the method of our invention over and over again, gradually increases due not only to the accumulation of acid washed out of the pickled stock, but also due to the fact that the hides being tanned take up more chrome than acid from the tan, and since chrome and acid are added to the solution in a constant ratio in maintaining the proper tan concentration, this condition also causes acid to accumulate. This is cared for as discussed below under the general subject of maintaining constant the composition of the tan liquor.

These acidity considerations, somewhat different from those of ordinary tanning methods,

make it possible to tan stock by our method that has not been pickled, thereby saving materialsacid and salt-and one operation. Stock for chrome tanning is pickled for two reasonsso that it can be kept for a period if desired before 5 tanning, that is to preserve the stock from the action of molds and bacteria, and also so that the stock going into tan will be uniform as to acidity, and so that at the start of the tanning period the relatively strong liquor will penetrate quickly into the interior parts of the hide. In carrying out the novel method of our invention, if stock is to be tanned at once after bating, the pickling may be omitted. In this case, the tan liquor must be maintained more acid than nor- 5 mal (basicity about .5 to .7) and it will be found best to complete the tanning in a second vat of dilute tan liquor having a high basicity--0.9 to 1.00. Bated stock to be tanned directly without pickling should be completely delimed and it is recommended that a higher ratio of ammonium salt to enzyme content be employed than is generally used in commercial bates so that deliming will be complete without the danger of depleting the stock by too much enzyme digestion. If de sired, commercial bates may be used and a small amount of acid (hydrochloric is preferred) may be added slowly and with care to the liquor in the paddle during the bating. This acid will assist in the deliming. Care should be taken not to make the liquor acid even locally.

As an example of the manner in which the method of our invention may be put into practice, the followingprocedure is described. The hides may be immersed in a pit or other container filled with the tan solution, and preferably having a removable false bottom and sides, so that all of the hides can be lifted out of the pit at once and be allowed to drain back into the pit; or, the hides may be hung on tenter hooks or sticks supported on a suitable frame or the like so that the entire lot of hides to fill one pit can be lowered by a suitable mechanical device into the pit and then be lifted out after the proper immersion period and be allowed to drain; or, the 45 hides may be thrown one at a time, as sorted for example, into the end of a long narrow pit or trough in which is slowly rotating a perforated cylinder nearly filling the trough and through which the hides slowly pass, the cylinder being 50 provided with flights which gently force the hides along the length of the cylinder, and the trough being provided with some mechanical device, for example an endless rubber belt with knobs fastened to the outside surface, to remove the hides as they arrive at the far end of the cylinder, the time of immersion in the tan liquor being determined by the length and speed of rotation of the cylinder, and the shape, number and position of the flights; or the hides or skins may be held out on a board or frame with tacks or toggles and tanned under tension, or, by any other means, such as handdipping in a pit or other container, which will completely immerse or contact the hides in or with a large volume of preheated concentrated tan liquor containing a very large excess'of dissolved chrome tan for a definitely determined and controlled period.

Approximately the same result is obtained by hanging the hides, grains, sides, splits or skins on tenter hooks, over sticks, under tension, or otherwise, supported on a suitable mechanical carrier or other device, and then carrying the hides singly or in lots to a point where they are contacted with a spray or shower, preferably from 75 each Room a number of jets operating under considerable pressure and arranged so that all portions of the hides and especially the flesh sides, are thoroughly sprayed and contacted with a large excess of preheated concentrated, recirculated tan liquor. This shower may be continuous for the period required to carry the desired amount of tan into the hides or it may be intermittent, bringing the same lot of hides back to the same shower or to other showers located on the carrier line or elsewhere.

Other methods of bringing the hides in contact with a very large excess of concentrated preheated tan liquor will be readily visualized by those well versed in the art of tanning, in fact this may be carried out in present day tanning equipment-wheels and paddles, preferably the latter since they do less harm to the stock, and in either case the amount of agitation reqired will be much less than is today used, since the large excess of preheated concentrated tan will quickly penetrate and tan the hides without agitation-and we do not wish to exclude any of these, although for most purposes we prefer those which do not involve the use of wheels and churns and the like, for these actually harm the leather by the continued pounding and involve high labor and power costs for their operation. The gentle swishing in the perforated cylinder and the gentle massaging by the pressure jets of tan solution described above, are advantageous since by an action similar to that occurring when a sponge is squeezed under water, this gentle massage causes an interchange between the more dilute and somewhat depleted liquor inside the hide in its intercellular spaces and the concentrated liquor of almost constant composition outside and in contact with the surface of the hide.

It can be readily seen that this method of tanning obviates many of the difiiculties inherent in the methods practiced heretofore and oifers certain other new and important advantages as well, some of which will be discussed below:- The tan liquor used may contain from 1 to 4 pounds per gallon of Tanolin A to 1 lb. per gal. of CrzOs) and the amount of liquor per hide (10 to gals. or more) will be many times greater than the volume used in prior art methods, and thus the ratio of available tan to hide substance will be some hundreds of times greater than any heretofore used. For example, in tanning side leather grains (split out of lime from cow-hides) approximately 200 gals. of tan liquor containing 4 lbs/gal. of Tanolin should be used to tan each 25 lbs. of drained pickled grain by our new method, whereas in the prior art short tan method not more than gal. of liquor would be used per 25 lbs. grain, and the concentration (at the start of tanning, only) would be about 4 lbs./ gal. of Tanolin. In other words, in carrying out our new method there are available per 25 lbs. cow-hide grain about 800 lbs. of Tanolin and in prior art methods there are available about 2 lbs. and at the outside 2% lbs. of Tanolin per 25 lbs. cow-hide grain. In the long liquor prior art method about the same quantity of tan is employed and more water is used so as practically to float the hides. Thus in either of the prior art methods the ratio of available tan to hide is only l/300th as great as it is in the method of our invention.

In carrying out our novel method, the removal of the amount of tan required for one 25# cowhide grain from the large amount present in 200 gals. of concentrated tan liquor does not deplete Therefore, the contanning period is practically as high as at the start, whereas in prior art methods the concentration of tan in contact with the hide rapidly drops to a low level as tanning proceeds, and almost at once approximates the equilibrium concentration for the hide in question, tanned to the particular point desired in the particular prior art process. It is the elimination of this unfavorable condition that accounts in large measure for the short tanning time required by the new method of our invention.

In order to obtain the many advantages of this new method of tanning, we have found it desirable to employ two or more tan vats or other units in series for contacting the stock with a large excess of preheated concentrated tan liquor so that the hides or skins may be lifted many or all at a time or continuously on a conveyor from one vat or other unit and placed for a period in the one or more other vats in the series. Thus, the solution in these several vats may be of different materials, concentrations, and acidities and be at different temperatures, or may contain water for rinsing and, if desirable, for conserving the materials so rinsed out of the stock.

For example, if a skin is lifted out of a tan solution of the concentration we employ, a much larger quantity of tan is removed by the skin than the skin can hold as combined tanthe surface of the skin is wet with the liquor and the pores are all fllled with it. In the case of a sheepskin from a 30 minute immersion in a 4# per gallon solution of Tanolin the total chrome picked up is more than 15% CrzOa on the basis of the dried skin. If such a skin is put through the regular washing and neutralizing process, all of this excess tan, above the 4 to 5% ClaOa which will be combined with the sheep skin, will be washed out from the skin and wasted and the cost of tanning by this method will become almost prohibitive.

We have found that if the skins removed from the preheated concentrated liquor after a necessary period of immersion and with or without draining during a short intermediate period, are immersed in a second vat of chrome liquor having a relatively low concentration, a number of economies and improvements are thereby brought about. In the first place, the skins removed from the second liquor will have in their pores and adhering to the surface of the skin, liquor of the more dilute concentration, so the excess removed from the second vat is therefore proportionately less. Next, the quality and concentrations of the tan in the two vats can be arranged so that approximately the right amount of chrome for tanning (skeepskins will always pick up an excess) is picked up out of the first high concen,- tration vat and so that this amount of chrome is redistributed and partially set in the second vat without losing an undue amount of chrome from the skin to the liquor in the second vat. Another important advantage may be credited to this procedurer-the chrome liquor in the first vat may be purposedly kept somewhat more acid than usual and the dilute solution in the second vat can be maintained as basic as desired; in fact, using lb. per gallon in the second vat. the chrome in this solution can be brought close to the precipitation point by the gradual addition of alkali, and the skin removed from a second tank which has been treated in this way will be found to have a pH of between 3-5, which is what is gen- TREATMENT t cH E T'cAL MODIFL GATION F TEXTILES c FIBERS,

erally desired for the subsequent coloring and fat liquoring operations. This procedure permits the use of a relatively acid pickle stock and/or a relatively acid tan in the first tank, and yet does not require the dangerous practice of adding bicarbonate solutions directly to the tan solution while the skins are being tanned. The low concentration chrome in the second tank permits a rinsing of the highly concentrated chrome adhering to the skin from the first tank, allows time for the transfer of acid from within the skin to the liquor of the second tank, changes the acidity of the skin to a higher pH value so that the tan is set and will not be removed when the skins are washed. This will not only maintain a higher average chrome content but will prevent detanning and embrittling of the grain.

This general procedure of using two or more tanks can be extended and will permit the rapid rinsing, neutralization and washing of the chrome tanned stock while still stretched out on the boards or frames, provided the stock is being tanned under tension, or without removal from the tenter hooks or sticks or other means of suspension if the stock is being tanned without tension. Moreover, the stock out of the first vat can be immersed in the second and more alkaline and more dilute tan solution at a relatively high temperature of 130 to 170 F. without harming the stock. Acid withdrawal from the skin and actual combination of the chrome tan with the hide substance at these temperatures is rapid. Immersion in a third tank of water at 180 or higher will very rapidly remove the excess tan and salt which is still in the skin. This further washing operation may be carried out more efiectively if the very dilute liquor or water in the vat is agitated and changed either at intervals or continuously.

Frequently, skins are retanned with a vegetable material following chrome tanning. In the case of sheepskins, we have completely chrome tanned in a vat of strong chrome liquor, rinsed, and set the tan in a second vat, washed in a third vat of hot water, retanned in a fourth vat of 50% quebracho, also hot (180 F.), and finally rinsed in a vat of 180 water, and have completed all of these operations in a period of 30 minutes. Thus, sheepskins for shoe lining stock or hat sweats or for embossing purposes can be stretched out on a board or frame and tanned, washed, neutralized, and retanned, and put into the drier ready for drying without removing from the frames in a period of from 30 minutes to one hour, thus saving not only time but much in the matter of labor.

The second vat in such a multiple vat system can be kept at the proper basicity .9 to 1.00 or more by a chemist, who can readily determine the amount of alkalipreferably a solution of caustic soda, as bicarbonate causes frothingthat must be cautiously added with constant stirring or plunging after each lot of skins is put through or at the end of each working day or shift or in a continuous manner. The tanner can also determine this in a practical manner by adding a solution of alkali cautiously with stirring until a chrome precipitate just persists. After standing over night (the solution being at a temperature somewhat above 110 F.) this precipitate will, in most cases,have dissolved. Employinga 5 x 5' x 5' tank, holding 20 frames with 40 sheepskins toggled out for tension tanning, and after 32 lots of frames have been placed in this second tank for 15 minutes each, the amount of caustic soda (70%) that must be added in order to bring back iii this figure but this indicates the order of mag 5 nitude.

It will be understood that no caustic soda or other strong alkali continues to exist as such after it has been added to the chrome solution. It is at once used up in neutralizing the acid in the vat, forming neutral sodium sulfate, for example, and when more is added it reacts with the soluble chromium sulfate and precipitates chromium hydrate or a basic salt, either of which is neutral. Thus, the acid of the liquor is reduced, but as long as any chrome remains dissolved, the solution does not become definitely alkaline even when caustic soda is added. This buifered solution is an ideal medium for reducing the acidity of the stock being tanned. Moreover, a certain 20 amount of this more basic liquor from the second vat, put back into the first vat for the reasons discussed below, will reduce the acidity of this first vat liquor which does tend to increase due to loss of pickle acid from the stock to the 25 liquor.

The chrome content of this second vat will gradually increase as will the neutral salt content. Some of this liquor can be put back into the first vat each day to make up for that removed by the 30 skins, etc., and to make up for the evaporation loss. Fresh Tanolin or other chrome tan must be added to the first vat each day or shift and this can be added to the liquor taken from the second vat or other dilute liquors or to some of the liquor so taken from the first or concentrated vat. To the second vat water is added to make up for all losses and evaporation, but despite these steps the concentration will gradually increase.

We have found that the chrome tan therein 40 contained can be purified and made available for reuse in the first vat in the following manner, and this procedure is also applicable to waste chrome iquors from any source. A small tank is nearly half filled with liquor from the second tank and 45 heated with a lead steam coil. Caustic soda is added until the chrome has precipitated, avoiding a large excess. This sludge is pumped through a filter press and the clear liquor containing the neutral salts is discarded. The small tank, after 50 rinsing, is now filled with heated strong liquor from the first vat and enough sulfuric or other strong acid is added to dissolve the precipitated chrome in the filter press. The amount is readily calculated by a chemist or can be determined by 55 a trial or two. This hot acidified liquor is then pumped through the press several times until the chrome sludge is dissolved and is then returned to the first vat of strong liquor. If desired, the neutral salt content of liquors in the strong vat 60 can be reduced in the same way. Dirt, fibres or any insoluble material which accumulate in the filter press may be discarded as the process is continued. Care should be taken to avoid allowing the precipitated chrome sludge to stand for 05 long periods since on long standing it dissolves with difficulty.

When it is desired to retan skins or hides with vegetable tan the multiple vat system is particularly eifective. ting the tan in the first two vats containing acid concentrated chrome liquor and dilute less acid chrome liquor, respectively, the skins are fully chrome tanned and will stand the boiling test.

After chrome tanning and set- 70 They may then be placed in a third vat of hot water approaching 180 F. to rinse out the excess chrome and salt and further reduce the acidity, and may be then placed in a fourth vat containing a strong hot solution of vegetable tan. Equal parts of quebracho extract and water in such a system will completely retan sheepskins in 5 or 10 minutes, while for heavier stock, a longer time of immersion may be required. With sheepskins, temperatures in excess of 180 may be employed, whereas with heavier stock somewhat lower temperatures are recommended. These high temperatures, permissible in retanning fully chrome tanned stock, make the time of retanning as well as the time of washing very short.

Following the retanning, the stock may be dipped in hot water or may be rinsed in any manner so that a clear grain will result. The stock, while still on the boards or frames, can be then dried in the manner described in U. S. Letters Patent No. 1,430,479, granted September 26, 1922 upon the application of Frank Wayland.

For very heavy hides for belting and sole leather, the same procedures may be followed, making due allowance for the difference in thickness which requires a longer time for penetration and using more caution with respect to the use of the higher temperatures.

The heaviest belting bends may be tanned by tensioning the pickled stock by mechanical means and fastening the tightly tensioned stock to a stout oak frame, and then immersing the whole assembly in a vat of chrome liquor (4#/gallon Tanolin) at F. At the end of four hours, penetration is complete although we prefer leaving such heavy stock in the vat over night so that much of the acid in the stock will difluse out into the liquor.

In the tanning of heavy leatherssole, belting stock, etc.,--the following advantages have been found to flow from the method of our invention: These leathers have been generally vegetable tanned because of the added weight that may be loaded into the stock by such treatment and because such leathers are sold by the pound; long periods of tanning are required60 to daysbecause of the need of preventing the well known case hardening effect when the stronger tan liquors are applied to the hide before a preliminary tanning is accomplished throughout the hide-or as the tanner says before the hide is struck through; since strong liquors cannot be used, the tanner has had heretofore two alternatives for speeding up his process-(l) raising the temperature, and (2) drumming or otherwise squeezing the stock at intervals under the surface of the tan solution in order to effect an interchange between the dilute liquor within the pores of the hide and the more concentrated liquor in contact with the outside layers of the hide. It is well understood that the rate of combination of vegetable tan with hide substance is relatively rapid, but that the rate of penetration of the large vegetable tan molecule into the pores of the hide or in through the cell walls is relatively slow. Higher temperatures increase the rate of penetration of these large molecules, as does alternately squeezing and releasing the hide much as an interchange is effected in a sponge so squeezed and released under water.

However, these prior art expedients of increasing the rate of penetration of the vegetable tan were limited in their application: If temperatures were raised before striking through with the vegetable tan, the stock was quickly injured, and

even after the preliminary vegetable tanning was accomplished the temperature to which the stock could be safely raised was not above 120 F., which is not high enough greatly to shorten the time a required for completing the tanning. Further- 5 more, when a vigorous squeezing, such as accomplished in a drum or a wheel, is employed, the stock is badly grained although the time required for tanning may be reduced. Drum tanned sole leather is always sold at a lower price 10 than the long process leather because of the broken up appearance of the grain, and more important perhaps because this drumming actually breaks up the fibres of the stock and the finished leather lacks the solidity and strength which is 15 an essential characteristic of this class of leather. There has been one other attempt made to shorten the tanning time and to avoid the above disadvantages, and that is by chrome tanning the stock first and then retanning with relatively 20 strong vegetable tans. It is well known that fully chrome tanned stock will stand boiling temperatures and, therefore, hot liquors for retanning may be used after chrome tanning. However, there are several disadvantages to this method. It has 25 been found that a heavy hide will take up just as much vegetable tan after chrome tanning as it will if no chrome tan is used, so all the costs of the chrome tanning are additional and there isno offsetting advantage excepting in time saved. 30 Furthermore, the leather is still lacking in solidity since drumming is necessary for both the chrome and vegetable tanning and the leather produced is soft and spongy and brings a low price compared to the leather produced by the long time 35 processes. And most disappointing of all, it is found very difficult indeed completely to chrome tan such heavy stock without a great waste of chrome tan and long drumming, and unless that is done the stock will not stand the boil test and 40 very hot liquors cannot be used.

In applying the new and improved method of our invention to the tanning of sole leather hides and belting stock, we have found that the same advantages apply-the chrome tanning is accom- 45 plished quickly without any drumming, there is a relatively small loss of chrome tan, the stock is completely chrome tanned, will stand the boil test and vegetable tan liquors almost boiling may be employed for retanning. The stock is not 50 broken up by drumming either for the chrome or vegetable retanning and it has the solidity of stock tanned by the prior art long time vegetable processes, and the retanning time itself is short even without drumming due to the high temperatures 55 employed.

In making belting leathers, we prefer before tanning to cut from the limed or pickled stock that part of the hide known as the belting bend and tension or stretch that part on a strong frame 9 with brass or bronze tacks or nails, or to hold it well stretched by any means, and while so tensioned apply to all exposed parts of the hide a great excess of preheated concentrated tan liquor-preferably by immersing the hide and any 55 necessary part of the supporting mechanism in a vat of such liquor. If desired, the whole hide may be so treated. In the case of ordinary heavy leather the hide prepared for chrome tanningand for this purpose the stock should be as com- 70 pletely delimed as possible-may be tacked out lightly on a board or toggled lightly on a frame, not for the purpose of stretching it but in order to hold it out flat, or the hide may be supported on a stick with or without tenter hooks, or by any 75 ULLHUIHHU UL UILIIUi I LUIU other means, so that one or many hides can be lowered into a suitable vat or pit and immersed in a large excess of preheated concentrated tan liquor. This liquor should contain about 4 pounds per gallon of Tanolin R, or an equivalent amount of other chrome or other metallic tan or mixture of tans, and should be heated to a point between F. and F. The basicity should be between 0.5 and 0.80. After a suitable immersion period depending on the thickness of the stock, the concentration and acidity of the liquor, and after the stock has remained in this liquor for at least one hour after penetration is complete, a period which in many instances may as well be over night, the stock is removed from the first vat. Then, after draining, it is immersed in a second vat of more dilute, less acid chrome tan liquor- A; pound of Tanolin per gallon and a basicity of from 0.95 to 1.00 or more. In fact, the chrome in this second tank should be kept near the precipitation point by the regular addition of alkali as described above. The stock should remain in the second vat for a period of hours until the pH of the stock in the interior as well as on the surface is about 4 or 4.5. In practice, we have found that if the stock is placed in the first vat early in the morning and is removed and placed in the second vat towards the end of the working day and is left in the second vat over night at a temperature well above 100 F., the stock will be well tanned.

At this point it is fully chrome tanned heavy leather and can be wash neutralized and finished and used as such. It is, in fact, a new article of manufacture, fully chrome tanned stock that has good solidity and that has not been broken up by drumming. Chrome tanned heavy leather of this solidity and strength that will stand the boil test,

has never before been made to the best of ourknowledge. Such leather in the form of a belt will wear very much longer than the ordinary chrome tanned belts now on the market and will actually outwear the best oak tanned (vegetable tanned) belts that are made. This is due to the fact that the leather is fully tanned; that can most readily be done by placing the hides in a third vat of hot water- F. in some casesand allowing the excess chrome and salts to wash out. The hides may then be placed in a fourth vat of strong hot vegetable tan material until the desired amount of retanning has taken place. For example a 50-50 mixture of commercial quebracho extract and water at 180 F. will completely penetrate heavy hides in a matter of hours-without any drumming, of course. If it is desired to go beyond this point, the hides may be left in this solution for a longer period or they may be immersed in another vat having an even higher concentration. It is understood that any vegetable or other tan or mixture of tans can be used in these vats for retanning. This is a matter that is well understood by tanners in general, and the amount and kind of tan are regularly adjusted in order to produce a desired result or to take advantage of the fluctuating prices on these many vegetable tan materials.

in the regular manner, although care must be taken not to drum the hides to such an extent that much of advantage of the new process is lost. However, before the hides are removed from the supporting medium, we prefer to rinse them from the last retan vat with hot water, and to add fat liquor and to bleach the grain with acid, neutralize and again rinse by immersing them in other tanks containing the proper solutions. Thus, labor and power throughout the tannery is reduced, and the hides are completely chrome tanned and, if desired, completely retanned to any point desired in two to seven days-without drumming, and whether retanned or not, a new and superior product is obtained-via, a heavy leather that is completely chrome tanned and which will stand the boil test and which has not been broken up by much drumming, and which, utilized either as a belt or a sole, will outwear any similar chrome tanned or retanned leather made by the prior art processes.

It should be pointed out that belting stock as ordinarily prepared will stretch in use and the belt must be removed and shortened generally several times during the first month or two of its life. Besides wasting valuable material, slackness of the belt causes a loss of production time in the plane and excessive slippage which slows down the plant operation. Efforts have been made to tan heavy leather under tension using vegetable tans so that there will be little or no stretch left in the stock used in the manufacture of belts. The difficulty with this plan is that the stretching frame or other supporting medium must remain in use with one hide or one belting hend for a period of weeks or months, and this tie-up of tannery equipment for such long periods makes this an expensive procedure. Either chrome tanned belts or retanned belts treated in accordance with the method of our invention and under tension will be stretch free. Since the tanning process is accomplishedin a short time, our improved method is of great economic importance in the production of stretch free belt leather.

We have further found that light skins. or even heavy hides to be used for belting stock or for a sole leather, can he tanned, with or without some preliminary treatment to protect the grain. by the method of our invention using vegetable tans, provided:--the skins or hides are contacted with a large excess of preheated concentrated vegetable tan liquor; the stock when contacted with the tan is well opened out so that penetration is rapid through the pores of the skin; the fleshing operation has been carried out sufflciently well to remove all adhering flesh and fat and the tight and fibrous nerve layer; and further provided that the concentation of the tan used is not allowed to drop appreciablv during the tanning period. In general, we prefer to protect the grain fibres, if not the fibres throughout the bodv of the hide, with some other tan material before contacting with the large excess of concentrated preheated vegetable tan but this is not always necessary.

In discussing the retanning of chrome tanned stock with vegetable tan we have described our preferred method. But we have found that if the grain is sprayed or swabbed with a small amount of a mineral tan, such as a strong chrome tan solution (basicity about 0.9), enough chrome tan will be picked up by the surface layers of the hide or skin to protect them from the case hardening eflect of the strong vegetable tan liquors. We have also successfully used for this purpose an alum pickle or synthetic tans such as Leukanol. Apparently this protection results from the fact that the surface layers are partially tanned by this superficial pretanning operation and are set in the position that they occupied when contacted with this tanning material. Thus, the advantage in having the skins or hides well set out and opened and preferably fastened out in this position, whether or not it is desired to tan actually under positive tension. In this way the pores and hair follicles are tanned while held wide open and the vegetable tan that is to be applied as a second step can penetrate rapidly. Actually, when hides are tanned in the ordinary manner in dilute solutions of vegetable liquors in rockers in the ordinary counter-current manner, the rate of penetration has been found to be nearly twice as fast as ordinary if the hides are tensioned out, and for the reason above stated. We have found that it is only necessary to permitthe vegetable tan to enter the main body of the skin or hide rapidly enough so that penetration throughout the stock is substantially complete before any large degree of tanning takes place, in order to make possible by our new method the elimination of the longtime tanning period generally necessary while tanning with vegetable tans.

To further this desirable end and to take advantage of the pressures of osmosis and diffusion both of which tend to force the dissolved vegetable tan into the hide which actsin part'as a semi-permeable membrane, and which are active only when an actual difference of concentration exists between the partially exhausted liquor within the pores of the hide and the concentrated liquor in contact with the outside of the hide, the concentration of the vegetable tan should not only be very high at the start of the tanning period but should be maintained substantially at this high level-and should not be permitted to fall until the stock has been well struck through with the vegetable tan. -Moreover, the temperature should be kept as high as possible and this point, of course, will depend upon the amount of pretanning, if any, that has taken place, on

the thickness of the stock, and on the extent of vegetable tanning that has taken place at any particular time. If care is taken to keep the flesh side of the stock well cleaned of nerve layer and actual flesh and fatty layers, and to keep the temperature as high as is possible without actually harming the stock, and in'maintaining the concentration of the vegetable tan at a high and approximately constant level, even heavy hides may be tanned in this way without the pretanning step with safety and without case-hardening and in a relatively short time. 1

As an example of tanning heavy sole leather hides by the method of our invention, the following procedure may be followed. The hides, completely delimed with lactic acid following a mild bate with oropon, are pickled with sulfuric acid and 7% salt, and are then tensioned out lightly on a board. The stock on the boards is then dipped for a few minutes in a solution of chrome tan (4#/gallon of Tanolin) at 105 F. and drained for 30 minutes. It is then immersed in a solution of commercial quebracho extract (75 to 95 parts of commercial extract to 25 to parts of water) at 110 F. until entirely struckv through and this may require about 2 days. The concentration of the quebracho tan is maintained approximately constant by frequent additions of extract.

Any other vegetable tan or mixture of tans 5 could be used in the process above outlined. For some purposes this would have been a sufilcient pick-up of vegetable tan but for others a longer time would be required in order to load the leather to the desired extent. When sufiiciently 10 tanned, the hides still on the boards are plunged into another vat of water or dilute liquor at 110 R, where the excess tan, particularly in the surface layers, is rinsed out, and conserved for reuse in the first concentrated vat after concentration by boiling, for example. Two rinse tanks in series are recommended. The stock is then ready for drying to crust or rough leather. It may be greased or stuffed as desired and finished.

If no pretanning is done, the well fleshed and well drenched hide is tacked out and preferably put under some positive tension and is immersed directly in the strong quebracho solution described above at a temperature of between 75 to 90 F. After from 8 to 24 hours, depending 25 on the tannery schedule, either the temperature of this tank is raised to 95 to 105 F., or the hides are lifted out and placed in a second similar solution heated to 95 to 105 F. In this way the benefit is secured of a continuously high concentration of tan and as high a temperature as is possible may be employed as tanning proceeds. The rinsing and finishing steps may be carried out as suggested above.

Throughout this disclosure we have insisted on a high tan concentration, maintained substantially constant throughout the tanning process and an enormous excess of tan in contact with the hides, and these are new and important improvements in the art of tanning never before practiced. We have described the application of our novel method to chrome tanning, to retan ning such chrome tanned stock with vegetable, and to vegetable tanning itself. Our examples of chrome tanning generally involve the use of Tanolin, a well known commercial tan, but are not limited to this particular prepared tan not to prepared tans in general. In fact, we have made the identical product by immersing the pickled skin in a concentrated solution of d chromate of soda and then in a solution of sodium bisulfite or other reducing agent, and then into a third solution of a very dilute chrome tan in which the basicity was kept high by alkali additions. In this case the second tank actually contained a strong solution of chrome tan plus a small amount maintained approximately constant of reducing agent and this second tank corresponds to the first tank of our preferredmethod In this case the first tank was for the 0 purpose of allowing the skin to pick up the necessary amount of chrome to tan the skin as dichromate, this dichromate being reduced to a chrome tan in the second tank. Thus, the tanner if he desires can use the cheaper dichromate as the 5 source of fresh chrome tan. The third tank in this system acted as the second tank in our preferred system.

Our examples frequently describe the use of tanks in which boards carrying the skins are immersed but we do not desire to be limited to these units and desire to claim any method of contacting the hide, skin, side, grain or split with an enormous excess of concentrated preheated tan liquor.

.TREATMENT 6i CHEMICAL MODIFL CANON F TEXTILES L FIBERS,

Moreover, Tanolin itself contains some aluminum as well as chrome, and we do not desire to limit ourselves to chrome and chrome tanning alone but contemplate using any tan which is contacted with the stock as a highly concentrated solution, applied in enormous excess, over and above the amount picked up by the stock in contact with the tan, and the concentration of which remains approximately constant during the tanning period.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. The method of tanning which is characterized by the step of contacting an untanned skin or hide with concentrated metallic tan liquor in such quantity that the residual tan in the used liquor represents more than the amount picked up by the tanned product.

2. The method of tanning which is characterized by the step of contacting a skin or hide with concentrated chrome tan liquor heated above 100 F. and in such a large quantity that the residual tan in the liquor exceeds substantially the amount removed by the tanned product.

3. The method of tanning in which a supply of chrome tan liquor is utilized over and over again in contact with successive lots of untanned skins or hides and is of such composition that less than 50% concentration is lost in tanning each lot.

4. The method of tanning in which chrome tan liquor is used to contact successive lots of skins or hides, the liquor being in quantity and composition such that its concentration may be maintained uniform by adding less than 50% make-up tan after the treatment of each lot.

5. The method of tanning characterized by the step of treating skins or hides with a metallic tan liquor of such composition that the concentration of the liquor in contact with the stock remains constant within 1% without the addition of fresh tan during the tanning process.

6. The method of tanning characterized by the step of treating skins or hides with a vegetable tan liquor of such composition that the concentration of the liquor which is in contact with the stock remains constant within 1% during the tanning process.

7. The method of tanning characterized by the step of treating skins or hides with a chrome tan liquor heated to between 100 and 140 F. and of such composition that the concentration of the liquor in contact with the stock remains constant within 1% during the tanning process.

8. The method of tanning characterized by the step of treating skins or hides with a chrome tan liquor of such composition and amount that, in

the liquor in contact with the stock, the concentration of chrome tan is not reduced more than 50% during the tanning process.

9. The method of tanning pickled hides or skins, which is characterized by the step of con- 5 tacting the stock with a concentrated chrome tan liquor in a quantity so enormously in excess of that required for the actual tanning process that the pH of the stock as removed from the tan liquor is not less than 3. 10

10. The method of chrome tanning which is characterized by the step of contacting the stock with a concentrated chrome tan liquor in a quantity so great that the basicity of the liquor is not substantially affected by the acid diffused into it from the stock.

11. The method of chrome tanning which is characterized by the step of contacting pickled stock with concentrated chrome tan liquor in a quantity so great that stock as removed from the tan liquor has a pH of not less than 3, without the addition of alkali to the tan liquor while the stock is in contact therewith.

12. The method of chrome tanning which is characterized by the step of contacting hides or skins with concentrated chrome tan liquor in a quantity sufiicient to receive acid diffused from the stock, maintain its normal basicity substantially unchanged and at the same time increase the pH of the stock to 3 or higher as withdrawn from the tan liquor.

13. The method of chrome tanning which is characterized by the step of contacting pickled stock having a pH of 3 or less with concentrated chrome tan liquor in such quantity as to change the pH of the stock, without the addition oi further alkalies to the tan liquor while in contact with the stock, to a pH at which the tan will set.

14. The method of tanning which is characterized by the step of contacting skins or hides with concentrated chrome tan liquor in such quantity that the basicity of the liquor remains during such contact at a suitable point to set the tan in the stock without the necessity of supplying additional alkali to the tan liquor.

15. The method of chrome tanning pickled stock, which is characterized by the step of contacting the stock with concentrated chrome tan liquor in such quantity that the acidity of the pickled stock is reduced during the process and its pH raised by difiusion of acid therefrom without substantially increasing the acidity .of the tan liquor.

16. The method of tanning which is characterized by the step of setting the tan while the stock is in contact with tan liquor containing in solution more chrome tan than the amount that has been picked up by the stock.

CLARENCE K. REIMAN. FRANK WAYLAND. 

